Reduction of residual stress and deformation in electron beam welding by using multiple beam technique

The electron beam could be controlled by magnetic field for fast deflection, in which way multi-beam could be produced by deflection technique. The multi-beams run simultaneously for material processing with different heat input and positions. Therefore, it is possible to adjust the thermal effects and optimize the process. In this paper, the generation of multi-beams in electron beam welding (EBW) was investigated, and the processes of EBW with multi-beams were also investigated by both the numerical simulation methods, i.e., finite element analysis (FEA), and the experiments. The result shows that the residual stress of EBW could be minimized by using the multiple beam technique, and at the same time the welding deformation could also be reduced with the optimized parameters.     

等离子喷涂ZrC基涂层逐道逐层沉积残余应力模拟与实验验证

采用商用ANSYS14.5软件, 依据复合梁增层力学模型, 采用逐道逐层累积模型模拟了C/C复合材料表面等离子喷涂ZrC基涂层沉积残余应力的特征, 分析了SiC过渡层、第二相(SiC, MoSi₂)和涂层厚度对ZrC基涂层残余应力的影响, 并进行了实验验证。结果表明, SiC过渡层有效缓解了涂层与基体的热失配应力。涂层体系的应力随着涂层厚度的增加逐渐减小, 符合应力松弛和叠加规律。在涂层内部的径向应力以拉应力为主, 基体中主要为压应力, 且在界面边缘存在压应力集中的极限区域, 易使涂层产生裂纹并沿界面扩展。该模拟采用逐道逐层累积的方法更逼近实际喷涂过程, 能更准确预测涂层的残余应力。     

等离子喷涂热障涂层残余应力的实验与模拟研究

分别采用真空和大气等离子体喷涂工艺在GH3128镍基高温合金基材表面制备CoNiCrAlY结合层和氧化钇部分稳定的氧化锆陶瓷层组成热障涂层。采用有限元模拟计算了涂层的残余应力, 研究了基材预热对打底层与陶瓷层界面应力分布的影响规律。结果表明, 预热基材可以显著地降低陶瓷顶层内部产生的残余拉应力。采用钻孔法测量了涂层中的残余应力并与模拟结果作定量比较, 结果表明: 有限元模拟计算结果与实验测量结果能较好吻合。     

A Method of Measuring Through‐Thickness Internal Strains and Stresses in Graphite

Abstract: Currently, the structural integrity of the nuclear graphite reactor components is evaluated using irradiated material properties data obtained from test reactors. These data are applied to numerical or mathematical models to assess the integrity of the graphite components. On the other hand, there is a need to measure internal strains and stresses in reactor core graphite and to explore the potential for in‐situ measurement. The deep hole drilling (DHD) technique is a semi‐destructive method for measurement of the through thickness residual strains or stresses. Previously the technique has been applied successfully to metallic and composite materials. In this paper, the method for internal strain and stress measurement in polygranular graphite is examined particularly when a significant volume fraction of porosity is present. Finite element simulation shows that the method can be used to measure internal stresses generated by a thermal gradient. On the basis of this a series of experiments have been conducted using Pile Grade ‘A’ (PGA) and PG25 filter graphite; with the latter being a surrogate for service exposed material. Tests were conducted to illustrate that the bulk mechanical response was linear elastic for both graphites and elastic moduli were measured. The DHD method was then used to determine internal strain and stress profiles through the materials. The results were compared with the strain data obtained from strain gauges bonded to the samples. Overall, there was an excellent agreement between the measured through‐thickness internal strains and stresses and the surface strains and stresses determined from the applied loading.     

硅表面钝化及对异质结太阳电池特性的影响

研究了不同的晶体硅表面钝化方法,测试分析了硅片的少数载流子寿命以及对晶体硅/非晶硅异质结(HIT)太阳电池性能的影响。发现适当时间的HF溶液处理、氢等离子体处理和表面覆盖约3nm的本征非晶硅层能有效提高硅片的少子寿命,从而提高HIT太阳电池的开路电压。对电池制备工艺综合优化后,得到了基于n型晶体硅的光电转换效率为16.75%(Voc=0.596V,Jsc=41.605mA/cm2,FF=0.676,AM1.5,25℃)的HIT太阳电池。     

C/C刹车材料摩擦表面层的微结构

采用透射电子显微镜（TEM）、 拉曼光谱（RMS）、 扫描电子显微镜（SEM）等手段, 研究了C/C复合材料在刹车过程中摩擦表面层微观结构变化, 建立了微结构模型; 利用有限元分析方法仿真了具有微凸体的试样在摩擦过程中的温度分布。研究表明: 摩擦表面除了形成一层数微米厚的摩擦层外, 还在摩擦层上不均匀地覆盖一层摩擦膜; TEM及选区电子衍射图（SAED）结果显示摩擦膜大部分区域为中等织构, 随着到外表面距离的减小, 织构度逐渐升高, 且在摩擦膜的最表面发现高石墨化度的区域; RMS同样证实摩擦表面存在局部高石墨化度区域, 摩擦过程中粗糙表面微凸体的最高温度远大于摩擦平面, 是导致摩擦表面应力石墨化的主要因素之一。      

整体化复合材料结构分阶段固化变形预报方法及其实验验证

通过测试分析了T800/环氧预浸料固化过程中性能参数的变化规律; 针对热压罐工艺条件下复合材料整体化结构分阶段成型的特点,提出了一种基于应力传递的分阶段固化变形的有限元模拟方法; 对于不同成型工艺的工型加筋壁板结构,将分阶段模拟得出的变形结果与测试结果进行了对比,并分析了成型工艺与变形量之间的关系。结果表明,本文中提出的模拟方法能够准确预报整体化结构的固化变形,变形与成型工艺密切相关,针对特定的结构与铺层,采用合理的工艺过程能够有效减小构件的固化变形。     

Comparison Between Using Longitudinal and Shear Waves in Ultrasonic Stress Measurement to Investigate the Effect of Post-Weld Heat-Treatment on Welding Residual Stresses

Fusion welding is a joining process widely used in the industry. However, undesired residual stresses are produced once the welding process is completed. Post-weld heat-treatment (PWHT) is extensively employed in order to relieve the welding residual stresses. In this study, effect of PWHT time and temperature on the residual stresses of a ferritic stainless steel is investigated. Residual stress distributions in eight welded specimens were measured by using an ultrasonic method. Ultrasonic stress measurement is a nondestructive method based on acoustoelasticity law, which correlates mechanical stresses with velocity of an ultrasonic wave propagating within the subject material. The ultrasonic wave employed could be longitudinal or shear wave produced by the longitudinal (normal) or transverse (shear) transducers, respectively. Ultrasonic stress measurements based on longitudinal waves use longitudinal critically refracted (L_CR) waves in this direction, while shear wave methods use an ultrasonic birefringence phenomenon. The results show that the effect of PWHT can be successfully inferred by both longitudinal and shear wave methods, but the former is found to be more sensitive to stress variation. Furthermore, the distribution of subsurface residual stresses is found to be more distinguishable when the L_CR method is employed.     

A fast numerical method of introducing the strengthening effect of residual stress and strain to tensile behavior of metal matrix composites

Thermal residual stress and strain (TRSS) in particle reinforced metal matrix composites (PRMMCs) are believed to cause strengthening effects,according to previous studies.Here,the representative volume element (RVE) based computational homogenization technique was used to study the tensile deformation of PRMMCs with different particle aspect ratios (AR).The influence of TRSS was assessed quantitatively via comparing simulations with or without the cooling process.It was found that the strengthening effect of TRSS was affected by the particle AR.With the average strengthening effect of TRSS,a fast method of introducing the strengthening effect of TRSS to the tensile behavior of PRMMCs was developed.The new method has reduced the computational cost by a factor 2.The effect of TRSS on continuous fiber-reinforced metal matrix composite was found to have a softening-effect during the entire tensile deformation process because of the pre-yield effect caused by the cooling process.     

A novel contact interaction formulation for voxel‐based micro‐finite‐element models of bone

Voxel‐based micro‐finite‐element (μFE) models are used extensively in bone mechanics research. A major disadvantage of voxel‐based μFE models is that voxel surface jaggedness causes distortion of contact‐induced stresses. Past efforts in resolving this problem have only been partially successful, ie, mesh smoothing failed to preserve uniformity of the stiffness matrix, resulting in (excessively) larger solution times, whereas reducing contact to a bonded interface introduced spurious tensile stresses at the contact surface. This paper introduces a novel “smooth” contact formulation that defines gap distances based on an artificial smooth surface representation while using the conventional penalty contact framework. Detailed analyses of a sphere under compression demonstrated that the smooth formulation predicts contact‐induced stresses more accurately than the bonded contact formulation. When applied to a realistic bone contact problem, errors in the smooth contact result were under 2%, whereas errors in the bonded contact result were up to 42.2%. We conclude that the novel smooth contact formulation presents a memory‐efficient method for contact problems in voxel‐based μFE models. It presents the first method that allows modeling finite slip in large‐scale voxel meshes common to high‐resolution image‐based models of bone while keeping the benefits of a fast and efficient voxel‐based solution scheme.     

Three‐dimensional measurement and cohesive element modelling of deformation and damage in a 2.5‐dimensional woven ceramic matrix composite

A cohesive element numerical model, which reproduces the three‐dimensional microstructure of a 2.5‐dimensional silicon‐nitrogen‐oxide fibre/fabric‐reinforced boron nitride ceramic matrix composite (SiNO/BN) is applied to simulate the failure of specimens that are observed in situ during diametral compression testing. Measurements of deformation by image correlation of two‐dimensional optical surface observations and three‐dimensional X‐ray computed tomographs are used to fit the simulation's elastic properties for the matrix and fibre tows. The observed patterns of damage nucleation and propagation are correctly simulated using a local tensile strain criterion.     

A numerical investigation of the connection between state of dispersion and percolation and its effect on the elastic properties of 2D random microstructures

The present work is dedicated to a numerical investigation of the connection between state of dispersion and percolation and its effect on the elastic properties of 2D random microstructures. The main objective consists in checking out the link between percolation and mechanical response in the context of a heterogeneous medium the reinforcements of which are not homogeneously dispersed. Besides, the influence of the stiffness of inclusions is also investigated since this could impact on the percolation effects. For these purposes, large samples of volume elements are generated according to the Monte Carlo method. We consider the low cost framework of 2D random grids which enables large and in-depth investigations. Besides, the spatial distribution of heterogeneities is simulated with the help of the 2-scale Boolean scheme of disks which is a powerful tool for modelling and studying several states of dispersion. The numerical results highlight beneficial mechanical reinforcements for a heterogeneous dispersion when the percolation phenomenon is enhanced. This improvement is highly sensitive to the stiffness of heterogeneities.     

胶粘剂厚度和弹性模量对胶焊接头应力分布的影响

采用三维弹塑性有限元分析法，数值分析了胶焊接头中胶粘剂厚度和弹性模量对胶焊接头内应力分布的影响。给出了五种没弹性模量和五种没厚度胶粘剂下，胶焊接头中焊点边缘和搭接区边缘的应力分布曲线，结果表明：胶粘剂的弹性模量低和厚度大时，胶层中的剪应力小而焊点边缘处正在明显的应力集中；胶粘剂弹性模量增大和胶层厚度减小时，接头中焊点边缘处应力减小而胶层中剪应力增大。在一定条件下，胶焊接头中采用易变形的胶粘剂对提高     

Investigating the static and dynamic tensile mechanical behaviour of polymer‐bonded explosives

The tensile properties of a polymer‐bonded explosive (PBX) were systematically studied by using quasi‐static and dynamic experiments. A non‐linear constitutive relation was developed to describe the tensile behaviour of the PBX. The tensile properties of the PBX under different strain rates and temperatures were measured in quasi‐static tests. The tensile behaviour of the PBX was found to exhibit high strain rate and strong temperature dependence, attributable to the large fraction of the polymer binder. To obtain the rational dynamic tensile results, a modified split Hopkinson tensile bar (SHTB) setup was designed such that the specimens were in dynamic stress equilibrium and deformed homogeneously at nearly constant strain rates. To characterise the viscoelastic behaviour, the master modulus curve was derived from the tensile stress relaxation tests at different temperatures. The non‐linear constitutive model was implemented in ABAQUS to predict the tensile behaviour of the PBX. The computational results were found to be in good agreement with the experimental results.     

On the local elastic–plastic behaviour of the interface in titanium/silicon carbide composites

The purpose of this study is to conduct a high-resolution nonlinear finite element analysis of the elastic–plastic behaviour of titanium/silicon carbide composites subject to transverse loading. This class of metal matrix composites is designed for the next generation of supersonic jet engines and deserves careful assessment of its behaviour under thermo mechanical loads. Three aspects of the work are accordingly examined. The first is concerned with the development of a representative unit cell capable of accurately describing the local elastic–plastic behaviour of the interface in metal matrix composites under thermal and mechanical loads. The second is concerned with the determination of the influence of mismatch in the mechanical properties between the inhomogeneity and the matrix upon the induced stress fields and the plastic zone development and its growth. The third is concerned with unloading and the role played by the interface upon residual stresses. It is found that the maximum interfacial stress in the matrix appears in the case involving cooling from the relieving temperature with subsequent applied compressive loading. It is also found that the mismatch in mechanical properties between the matrix and the inhomogeneity introduces significant changes in the stress distribution in the matrix. Specifically, it is observed that the maximum radial and tangential stresses in the matrix take place at the interface. The plastic deformation of the matrix leads to a relaxation of these stresses and assists in developing a more uniform interfacial stress distribution. However, the matrix stresses and the resulting equivalent plastic strains still reach their maximum values at that interface. The results show similarities in the patterns of the interfacial stress distribution and plastic zone development for all ranges of fibre volume fractions and loading levels examined. However, they also show marked differences in both the magnitude and patterns of matrix stress distribution between the adjacent inhomogeneities as a result of interaction effects between the fibres.     

Contact stress and residual stress in the nose rail of a high manganese steel crossing due to wheel contact loading

Fatigue failure of a high manganese steel crossing is related to its internal crack initiation and growth, which is affected significantly by the magnitude and distribution pattern of contact stress and residual stress in the crossing. Considering the actual service conditions of a crossing and the accuracy requirement for numerical calculation, a whole model of wheel/crossing/ties and a partial model of wheel/crossing are established using elastic‐plastic finite element method. The distributions of contact stress fields and residual stress fields due to wheel contact loading are studied. The effect of train speed on the residual stress in the nose rail is discussed. The contact stress field shows regular contours in the cross‐section of nose rail and decreases remarkably with increasing distance of the wheel‐crossing contact position. The maximum contact stress is located at the contact surface between wheel and crossing. The maximum residual stress is located at a position of 1.5‐2.0 mm below the surface of the nose rail, rather than at the contact surface of wheel and crossing. In a failed high manganese steel crossing, the dense cracks mainly were observed neither at the position of maximum contact stress (the contact surface between the wheel and the crossing), nor at the position of maximum residual stress (1.5‐2.0 mm below the surface of the nose rail), but around the depth of 0.8‐1.0 mm from the worn surface, which is between the position of maximum contact stress and the position of maximum residual stress. It indicates that the combined effects of the maximum contact stress and the maximum residual stress play important roles in fatigue crack initiation in the nose rail. The size of high residual stress region increases with the increase of the train speed. The maximum residual stress in the nose rail increases remarkably with the increase of the train speed.     

Effects of welding speed on the multiscale residual stresses in frictionstir welded metal matrix composites

The effects of welding speed on the macroscopic and microscopic residual stresses(RSes) in friction stir welded 17 vol.% SiCp/2009 Al-T4 composite plates were studied via neutron diffraction and an improved decoupled hierarchical multiscale modeling methods. Measurements showed that the macroscopic and total RSes had the largest variations in the longitudinal direction(LD). Increasing the welding speed led to higher values of measured LD macroscopic and total RSes in the matrix. The welding speed also significantly influenced the distributions and magnitudes of the microscopic RSes. The RSes were predicted via an improved hierarchical multiscale model, which includes a constant coefficient of friction based thermal model. The RSes in the composite plates before friction stir welding(FSW) were computed and then set as the initial states of the FSW process during modeling. This improved decoupled multiscale model provided improved predictions of the temperature and RSes compared with our previous model.     

A thermo-viscoelastic approach for the characterization and modeling of the bending behavior of thermoplastic composites

Currently, there is no established and cost-effective method for the bending characterization of continuous fiber reinforced thermoplastic composites. Isothermal mechanical testing techniques are time and labor-intensive and deliver information only about distinct points of the temperature-dependent property curves. In this study, Dynamic Mechanical Analysis (DMA) as well as novel rheometer-based bending experiments were performed to assess temperature-dependent and viscoelastic behavior. On the basis of the experimental results a new method was defined and validated for the efficient characterization of temperature-dependent elastic bending behavior via DMA. Furthermore a linear viscoelastic material model was derived from DMA experiments by means of time–temperature superposition. As the material behavior proved to be of a highly viscoelastic nature, a method was developed to calibrate a material model, the parallel rheological framework, implemented in Abaqus.     

Simulation of Crack Propagation in Functionally Graded Materials Under Mixed-Mode and Non-Proportional Loading

Automatic simulation of crack propagation in homogeneous and functionally graded materials is performed by means of a remeshing algorithm in conjunction with the finite element method. The crack propagation is performed under mixed-mode and non-proportional loading. Each step of crack growth simulation consists of calculation of mixed-mode stress intensity factors by means of a novel formulation of the interaction integral method, determination of crack growth direction based on a specific fracture criterion, and local automatic remeshing along the crack path. The present approach requires a user-defined crack increment at the beginning of the simulation. Crack trajectories obtained by the present numerical simulation are compared with available experimental results.     

A parametric study of fiber volume fraction distribution on the failure initiation location in open hole off-axis tensile specimen

We present a model to study the effect of the spatial distributions in fiber volume fraction on the failure initiation location in the open hole off-axis tensile. These variations are introduced with a micromechanical enhancement dehomogenization process. Good agreement is obtained between our predicted failure locations and experimental results by considering a failure criterion based on the effective shear strain in the matrix. The predicted failure angle distribution is in good agreement with the experimental results when the variability in the fiber volume fraction is included in the simulations.